Method of regenerating waste pickle liquor

ABSTRACT

A PROCESS FOR REGENERATING AQUEOUS SULFURIC ACID PICKING LIQUOR THAT CONTAINS RESIDUAL IRON-SULFUR COMPOUNDS CHARACTERIZED BY THE STEPS OF (1) OXIDIZING THE IRON-SULFUR COMPOUNDS IN AT LEAST A PORTION OF THE LIQUOR BY RAISING ITS PH LY NEUTRALIZING THE PORTION OF THE LIQUOR BY RAISING ITS PH TO AT LEAST 2 WITH THE ADDITION OF A SOLUBLE CAUSTIC MATERIAL, PREFERABLY ONE THAT CONTAINS A CATION WHOSE REACTION PRODUCT AFTER THE TWO STEPS FORMS A PECIPITATE AT A PH OF 2 AND HIGHER; (3) SEPARATING THE PRECIPITATES FROM THE REGENERATED PICKLING LIQUOR; AND (4) RETURNING THE REGENERATED PICKLING LIQUOR TO PICKLING SERVICE. ALSO DISCLOSED ARE SPECIFIC PREFERRED MATERIALS AND PROCESSES EMPLOYING ADDITIONAL STEPS.

3,816,593 METHOD OF REGENERATING WASTE PICKLE LIQUOR William L. Johnson,1727 Chip-n-Dale, Arlington, Tex. 76010, and William R. Massey, FortWorth, Tex.; said Massey assignor to said Johnson No Drawing. Filed Nov.30, 1971, Ser. No. 203,421 Int. Cl. C01b 17/90; C01g 49/02 U.S. Cl.423-140 9 Claims ABSTRACT OF THE DISCLOSURE A process for regeneratingaqueous sulfuric acid pickling liquor that contains residual iron-sulfurcompounds characterized by the steps of (1) oxidizing the iron-sulfurcompounds in at least a portion of the liquor; (2) partiallyneutralizing the portion of the liquor by raising its pH to at least 2with the addition of a soluble caustic material, preferably one thatcontains a cation Whose reac tion product after the two steps forms aprecipitate at a pH of 2 and higher; (3) separating the precipitatesfrom the regenerated pickling liquor; and (4) returning the regeneratedpickling liquor to pickling service. Also disclosed are specificpreferred materials and processes employing additional steps.

BACKGROUND OF THE INVENTION (1) Field of the invention This inventionrelates to a method of regenerating and returning to pickling servicethe sulfuric acid of a pickling liquor that also contains theiron-sulfur compounds resulting from pickling operations.

(2) Description of the prior art Disposal of waste pickling liquor hasbeen and still is an expensive and troublesome problem to variousindustries, particularly those industries concerned with cleaning asurface containing iron or the like preparatory to deposition of anothercoating thereon, as in galvanizing. Many processes have been proposedthat would use or regenerate the waste pickling liquor; but theseprocesses have not been totally satisfactory, being either impracticalin operation or economically infeasible in the particular industry.Consequently, mills or the like have had to resort to neutralizationbefore discharge or other disposal methods. With the current emphasis onimproving the eco ogy, the processes for neutralizing the undesirableproducts have become more expensive, frequently requiring shut-down,pumping and cleaning up to prevent pollution. A good discussion of theprior art pickling practice is to be found in U.S. Pats. Nos. 3,387,927;2,946,659; 2,739,040; and even back as early as 1,384,974. Relatedchemical processes are also discussed in U.S. Pats. Nos. 1,636,296;1,606,470; and 1,047,826. The process of these patents oxidized the ironions from their lower +2 to a higher +3 oxidation number. A studyofthese prior art processes indicate that the processes, thoughtheoretically sound, never achieved the full usefulness that they shouldhave. We believed the primary reason for the lack of a totallysatisfactory solution was that these processes did not take advantage ofthe effect of pH on the solubility of reaction products. Specifically,these processes attempted, with logical basis, to keep the pH of thepickling solutions as acid as possible; normally in the range of about0.5-1.0. Consequently, the prior art processes had to advert to asupplemental operation such as heating or the like to convert thereaction productsto a precipitate. The additional conversion step wasexpensive, time consuming, and not readily adaptable to the oxidationreaction step. It is deemed probable that the prior workers UnitedStates Patent 3,816,593 Patented June 11, 1974 "ice in this fieldoverlooked the effects of pH on solubility, because conventionalsolubility data is given at near neutral conditions, not in highly acidconditions.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide a process for regenerating waste pickling liquor in whichdefects of the prior art processes are obviated.

It is a specific object of this invention to provide a process in whichthe alkalinity of the treated liquor is increased by a factor of atleast 10 in order to obtain precipitation of at least the oxidizedreaction product of the iron-sulfur compounds that contaminate pickleliquor; thereby enabling the regenerated pickle liquor, without theiron-sulfur compounds that are removed, to be returned to picklingservice; and enable cleaning up the pickling solution without pollutinga surrounding ambient environment, or otherwise disturbing theecological conditions that prevail.

These and other objects will become apparent from the followingdetailed-descriptive matter. H

In accordance with the invention, contaminated, or degenerated, sulfuricacid pickle liquor is regenerated and restored to service by thefollowing multi-stepprocess. As one of the steps, the iron-sulfurcompounds in the degenerated pickle liquor are oxidized by contacting atleast a portion of the liquor with an oxidizing agent that is strongenough to oxidize the iron-sulfur compounds. The oxidizing agent isemployed in an amount that is at least stoichiometrically equivalent tothe iron ions to be removed from the portion of the pickling liquor.

As another step, the portion of the pickling liquor is partiallyneutralized by adding a soluble caustic material in an amount suflicientto bring the pH of the portion of the liquor to at least 2. A minimum pHof 2 is required in order for the oxidized iron-sulfur com-- pounds toform precipitates that can be separated from the liquor. Preferably, inorder to prevent a high concentration of a strong electrolyte in theliquor, the caustic material contains a cation whose reaction productformed after the oxidation and partial neutralization steps also forms aprecipitate at a pH of 2 and higher. It can be seen that the pH greaterthan 2 actually effects a hy droxide ion concentration, or alkalinity,that is more than 10 times that normally attending the pickling liquor.The step of deliberately partially neutralizing a portion of thepickling liquor produces a high degree of precipitate formation andenables separating and returning the regenerated pickling liquor toservice. Even we have been surprised by the unexplained and unexpectedprecipitation of both the oxidized iron-sulfur compounds and thereaction product of the cation of the caustic material.

As a third step, the precipitates, both the oxidized ironsulfurcompounds and the reaction product of the cation of the causticmaterial, are separated from the portion of the pickling liquor, whichthen is a regenerated pickling liquor. The separation may be by any ofthe conventional processes.

As a fourth'step, the regenerated pickling liquor is returned to thepickling service.

The described process may be carried out as a batch operation, the stepsbeing carried out in either a separate vat holding a portion of thepickling liquor, or the entire storage tank for small systems. On theother hand, the process may be carried out as a continuous process. Thecontinuous process may be employed to treat either full stream or sidestream. For example, it may be carried out as a treatment of about 10gallons per minute for side stream operation in which an entire picklingoperation can have its pickling liquor regenerated by operation of thetreatment plant periodically for about 2 to 3 days- On the other hand, asmaller treatment may be carried out with a smaller side stream operatedcontinuously.

This invention is useful for regenerating, or recovering, waste picklingliquors from steel treating processes as well as other processes whichemploy pickling liquors; for example, regenerating waste liquors fromelectroplating processes. The invention has several advantages. Forexample, impurities such as zinc impurities have frequently requiredre-running because of the high toxicit thereof. These impurities arealso precipitated in the process. The impurities constitute a relativelysmall percentage of the precipitate, so the precipitate is primarilyiron, and it may be sold for recycling to produce iron ingots. Any otherconstituent; such as, the metal of the cation of the caustic material;may be recovered also, if present in a feasible concentration. Theprimary benefit of the invention, however, is that it prevents having toshut down the particular operation; for example, having to shut down alarge scale galvanizing operation; in order to clean up the picklingliquor and prevent a polluting discharge that will create legal andecological problems. Moreover, the invention alleviates problems whichrequire separate pumping systems for cleaning up the process, asfrequently required in conventional processes.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS In carrying out the inventiondescribed briefly hereinbefore, the oxidizing agents that are employedare those which are strong enough to oxidize the iron-sulfur compounds.Satisfactory oxidizing agents include the halogens; derivatives of ahalogen, such as bromine water; concentrated nitric acid; and hydrogenperoxide. There may be health hazards associated with employing certainof the oxidizing agents; such as, fluorine, chlorine, and the lessstable derivatives like the perchlorates. By their very nature, thestrong oxidizing agents present some hazard and require specialprecautions. The procedures embodying the precautions, however, are wellknown, have been carefully delineated in safety manuals, and need not bedescribed in detail herein. A particularly preferred oxidizing agentcomprises chlorine, since it may be readily contacted with the solutionin either a batch process or a continuous process. For example, thechlorine may be first dissolved in water and then reacted with the ironions in the pickling liquor; or it may be contacted directly wth thepickling liquor in any of the conventional processes. Specifically, thegaseous chlorine can be bubbled through perforated pipe in the bottom ofa vessel in a batch process; or flowed into the bottom of a contactingtower for contacting the downcoming liquor in a continuous process.Ordinarily, it is not necessary to employ an expensive bubble traytower, since a tower that is packed with Berl saddles, Raschig rings, orthe like will afford a satisfactory contacting tower. If one of thechemicals that is equivalent to the chlorine; that is, another of thestrong oxidizing agens; is employed, the contacting procedures that areconventional for that particular oxidizing agent will be employed in theoxidation step.

The caustic material that is employed to bring the pH of the liquor toat least 2 must be soluble in the liquor such that it will raise thehydroxide ion concentration to the level equivalent to a pH of at least2. Preferably, the caustic material contains a cation whose reactionproduct formed after the oxidation and partial neutralization stepsforms a precipitate at the pH greater than 2. Ordinarily, this willrequire a cation which is at least divalent. Typical of such solublecaustic materials are calcium hydroxide and magnesium hydroxide.Particularly preferred is magnesium hydroxide. The reasons for itsunusually eificacious performance are not completely understood. Thesolubility of chemical compounds in highly acidic solutions is stillexploratory in nature. The chemical kinetics of the reaction of themagnesium compounds in pickling liquor are uncertain. Chemical equationsare set forth hereinafter in what is believed to be the explanation ofthe success of the process. Of course, caustic, materials containing thesoluble monovalent cations can be employed where a build-up of themonovalent cation can be tolerated.

As indicated hereinbefore, the exact phenomena which explains theefficacy of the process is not clearly understood. The followingequations are given by way of tentative explanation, not by way oflimiting the process; since the process works, whether or not theequations that are hypothesized are accurate. The iron-sulfur compounds,in soluble form at the low pH have their ions oxidized to the highestvalence of the iron; that is, iron ions having a valence of a positive3, sometimes referred to as iron (HI), all in accordance with Equation I(in ionic form):

The addition of the magnesium hydroxide effects precipitation of theiron hydroxide in accordance with Equation II:

A surprising phenomena that has been observed and that is not as clearlyunderstood is the subsequent precipitation of a major portion of themagnesium that is added. It may be that the precipitation effects acontinuing movement of the reaction in accordance with Equation III tothe right, even though the predominant reaction rate is to the left sideas follows:

In any event, the solubility phenomena which result in the precipitatingof the iron and magnesium compounds is only experienced at a pH greaterthan 2; although we have found that magnesium ions alone are removed bythe oxidation step at pHs as low as 0.4. Expressed otherwise, asatisfactory proportion of the iron and magnesium compounds areprecipitated when the pH is as great as 2. The iron hydroxide can beprecipitated substantially quantitatively at a pH of about 4 or higher.In one example, we have found that at a pH of 5 the concentration ofiron ions have been lowered from about 17,000 parts per million (ppm) toabout 1 ppm.

The oxidizing agent is employed in an amount that is at leaststoichiometrically equivalent to the iron ions to be removed.Preferably, an excess of the oxidizing agent is employed, particularly,where the efiiciency of contacting the liquor is such that it effectsless than percent contact.

The soluble caustic material is employed in an amount sufiicient tobring the pH of the portion of the liquor to a pH that is at least 2. Ifthe maximum precipitation is desired, an amount of soluble causticmaterial is employed that will raise the pH to a level of about 5.

The steps of separating the precipitates from the re generated picklingliquor and returning the regenerated pickling liquor to the picklingservice may be carried out by any of the conventional separationprocesses. For example, a settling basin, with or without additives, maybe provided such that the heavy salt precipitates will settle to thebottom and the supernatant liquid drawn from a quiescent zone. On theother hand, the iron compounds may form floc-like precipitates such thatseparation will be more nearly complete if a filtering operation isdone. In such an event, the supernatant liquid from a quiescent zone ofa treatment, or settling, vessel is flowed through a filter where theprecipitate is removed. The filtrate is regenerated liquor that isreturned to pickling service. That portion of the precipitate which istrapped by the filter may be subsequently removed by backwashing thefilter. Ordinarily, a settling vessel will also have a concentratedslurry of precipitate which can be blown down, or sent to a recoverypit, to recover the precipitate without clogging the filter. If desired,centrifugation, with or without additives, may be employed to separatethe precipitates from the pickling liquor.

The process may be carried out in any order of steps desired. Forexample, the soluble caustic material may be added first to raise the pHof the portion of the liquor to be treated and thereafter the portion ofthe liquor contacted with the oxidizing agent to oxidize the ironsulfurcompounds. On the other hand, the oxidation of the iron-sulfur compoundsmay be carried out first, as by bubbling the chlorine through theliquor; and subsequently, the soluble caustic material added to raisethe pH. If desired, the process may be carried out by simultaneouslycarrying out both steps; that is, adding the soluble caustic materialwhile simultaneously contacting the liquor with the oxidizing agent, asby bubbling the chlorine therethrough.

In the event that the liquor is contacted with the oxidizing agent firstand the soluble caustic material added thereafter, an additional step ofcontacting the efliuent from the partial neutralization step with theoxidizing agent may be carried out to further precipitate the cationreaction product; that is, the reaction product formed by reaction ofthe cation of the soluble caustic material during the oxidation andpartial neutralization steps. For

example, an additional chlorinator may be employed, or

the additional step of chlorinating the supernatant liquid employed, toeffect further precipitation of the cation reaction product; such as,magnesium hydroxide, as in accordance with Equation III.

The process of the invention is enhanced in its eflicacy by periodicallyconcentrating the sulfuric acid by removal of the water therefrom. Thewater may be removed from the sulfuric acid by any of the conventionalprocesses; for example, by evaporating the water from the dilutesulfuric acid to form a more concentrated solution. The water that isremoved does not, of course, pollute the atmosphere or the environment,and does not require special handling, as did the waste liquor that hadto be disposed of previously. As indicated hereinbefore, theprecipitate, or sludge, containing the precipitates is a desirablecommercial product and does not present disposal problems.

Having thus described the invention, it will be understood that suchdescription has been given by way of illustration and example and not byway of limitation.

What is claimed is:

1. A method of regenerating aqueous sulfuric acid pickling liquor thatcontains residual iron-sulfur compounds comprising ferrous sulfatecomprising the steps of:

(a) oxidizing said iron-sulfur compounds comprising ferrous sulfate toferric sulfate by contacting at least a portion of the liquor with anoxidizing agent selected from the group consisting of the halogens,concentrated nitric acid and hydrogen peroxide; said oxidizing agentbeing present in an amount that is at least stoichiometricallyequivalent to the iron ions to removed; then (b) adding to said portionof said liquor a soluble caustic material selected from the groupconsisting of calcium hydroxide and magnesium hydroxide in an amountsufiicient to bring the pH of the portion of the liquor to at least 2 toform precipitates comprising ferric hydroxide and said caustic material;

(c) separating the precipitates from the regenerated pickling liquor;and

(d) returning the regenerated pickling liquor, sans the removediron-sulfur compounds, to pickling service.

2. The method of claim 1 wherein said oxidizing agent is employed in anamount that is in excess of the stoichiometric equivalent of the ironions to be removed.

3. The method of claim 1 wherein said soluble caustic material is addedin an amount suflicient to bring the pH of the portion of the liquor toat least 4.

4. The method of claim 3 wherein said pH is brought to at least 5.

5. The method of claim 1 wherein said oxidizing agent is selected from agroup consisting of chlorine, bromine Water, concentrated nitric acidand hydrogen peroxide.

6. The method of claim 5 wherein said oxidizing agent is chlorine.

7. The method of claim 1 wherein said caustic material is magnesiumhydroxide.

8. The method of claim 1 wherein an additional step of chlorinating saidportion of said liquor is employed subsequently to step(b) to furtherprecipitate the caustic material.

9. The method of claim 1 wherein said oxidizing agent is chlorine andsaid caustic material is magnesium hydroxide.

References Cited UNITED STATES PATENTS 2,416,744 3/ 1947 Francis 423-1441,824,936 9/ 1931 Travers 423144 X 1,879,577 9/1932 Stauf et al 423l473,164,463 1/ 1965 Graham et al 423l47 2,433,458 12/ 1947 Kahn et al423l47 X 3,387,927 6/ 1968 Goldberger 4235 31 3,549,321 11/1970 Everett423140 1,146,071 7/1915 Hoffman 423146 X I. COOPER, Assistant ExaminerUS. Cl. X.R. 42353 1, 632, 639

